Colorants that are typically used in inkjet inks include dyes or pigments. While dyes are more commonly used in inkjet inks, images printed with dye-based inkjet inks are generally less waterfast, lightfast, and resistant to ozone than images printed with pigment-based inkjet inks. However, the pigments are typically insoluble in an ink vehicle of the inkjet ink and form discrete particles that clump or agglomerate if they are not stabilized in the inkjet ink. To prevent the pigments from agglomerating or settling out of the ink vehicle, the pigments are uniformly dispersed in the inkjet ink and stabilized in the dispersed form until the inkjet ink is used for printing. The pigment is typically present in the inkjet ink in a distribution of particle sizes, which is selected based on performance attributes, such as stability, gloss, optical density (“OD”), and the like.
Polymeric dispersants are used to disperse particles of the pigment in the ink vehicle through non-covalent interactions. The polymeric dispersant binds to a surface of the pigment, providing stability to the pigment dispersion. Alternatively, the pigment particles are modified by covalently attaching solubilizing moieties, such as polymeric, ionic, or organic groups, to the surface of the pigment. The surface-modified pigments or self-dispersed pigments expand the potential range of chemical and physical diversity of the pigment.
An inkjet ink having a first pigment, a second pigment, and benzylmethacrylate as a polymer dispersant has been disclosed. The inkjet ink forms an ink dot having a uniform and high density and proper dot size. The first pigment is a self-dispersed pigment that has anionic groups bonded to its surface while the second pigment is dispersed by the polymer dispersant.
An inkjet ink having good color development has also been disclosed. The inkjet ink includes a first colorant and a second colorant. The first colorant is a first pigment that has carboxyl, carbonyl, hydroxyl, or sulfone groups attached to its surface and is dispersible or dissolvable in water without a dispersant. The second colorant is a pigment that requires a dispersant, such as a polymeric dispersant or surfactant, in the ink vehicle.
Inkjet inks are printed on a wide variety of print media, such as plain paper print media (“plain paper”) or specialty print media. Specialty print media include glossy print media, such as porous print media or swellable print media, and transparencies. Each type of print media is optimized for different end uses and has diverse chemical and physical properties. Therefore, dispersion chemistry and particle size of the pigment-based inkjet ink are optimized to achieve acceptable print quality across the different types of print media. However, in order to achieve the acceptable print quality across the different print media, compromises in performance attributes are made on each type of print media. As such, while the inkjet ink provides acceptable print quality across the different print media, the inkjet ink does not provide optimal print quality on each of the print media. In addition, it is difficult to use a particle size distribution of pigments having optimal performance attributes across these diverse print media types.
For instance, to enhance OD of the printed image on plain paper, a pigment used in a black, pigmented inkjet ink would typically have a high structure, a fast rate of agglomeration, and a large aggregate particle size. As used herein, the term “high structure” refers to a pigment having a dibutylphthalate absorption number (“DBP number”) greater than approximately 100 ml/100 g, per ISO 4656 or similar test methods. In contrast, “low structure” refers to a pigment having a DBP number ranging from approximately 40 ml/100 g to approximately 70 ml/100 g. However, if the black, pigmented inkjet ink described above was printed on a glossy print medium, the printed image would have poor gloss, a high surface roughness, and reduced durability.
Chemical and physical diversity is also encountered within a particular type of print medium. For instance, distinct chemical and physical domains are present in low-cost, plain papers, especially in plain papers having a high content of recycled materials. The domains range in size from approximately one micron to approximately several hundred microns and include domains of cellulose, sizing agents, or inorganic pigments, such as calcium carbonate. Current drop volumes of thermal and piezo inkjet printers yield dot sizes on the same scale as these domains. Therefore, when the inkjet ink is applied to the print medium, the inkjet ink is exposed to different environments depending on the domains that it encounters. Since each of the domains have different chemical and physical properties compared to other portions of the print medium, the inkjet ink interacts differently with the domains.
Pigments are also widely used in the paint industry. To produce a desired paint color, multiple pigment dispersions are typically combined. However, when more than one pigment dispersion is used, the pigments may separate from each other, which is an undesirable phenomenon known as floating or flooding. Floating refers to an uneven distribution of the pigments after the paint is applied and results in a mottled, blotchy, or streaked color variation in the applied paint film. Floating is caused by using different particle size pigments in the paint, using more than one type of pigment, different pigment flocculation rates, or gradients in surface tension, density, or temperature. Flooding produces a uniform color of the paint film but variations in color occur throughout the coating thickness caused by different rates of pigment settling in the paint film. The pigment settling rates are due to differences in pigment size, density, shape, or degree of flocculation. To prevent floating or flooding, anti-floating and anti-flooding additives are added to the paint.